The invention relates to drained-cathode cells for the electrowinning of aluminium from alumina dissolved in a molten fluoride-containing electrolyte having sidewalls resistant to molten electrolyte, and methods of operating the cells to produce aluminium.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite-graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode. The side walls are also covered with prebaked anthracite-graphite carbon plates.
To increase the efficiency of aluminium production numerous drained-cathode cell designs have been developed, in particular including sloping drained cathode surface, as for instance disclosed in U.S. Pat. No. 3,400,061 (Lewis/Altos/Hildebrandt), U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Stephen), U.S. Pat. No. 5,368,702 (de Nora), U.S. Pat. No. 5,683,559 (de Nora), European Patent Application No. 0 393 816 (Stedman), and PCT application WO99/02764 (de Nora/Duruz). These cell designs permit reduction of the inter-electrode gap and consequently reduction of the voltage drop between the anodes and cathodes. However, drained cathode cells have not as yet found significant acceptance in industrial aluminium production.
It has been proposed to decrease energy losses during aluminium production by increasing the thermal insulation of the sidewalls of aluminium production cells. However, suppression of the thermal gradient through the sidewalls prevents bath from freezing on the sidewalls and consequently leads to exposure of the sidewalls to highly aggressive molten electrolyte and molten aluminium.
Several proposals have been made in order to increase the sidewall resistance for ledgeless cell operation. U.S. Pat. No. 2,915,442 (Lewis) discloses inter-alia use of silicon carbide or silicon nitride as sidewall material. U.S. Pat. No. 3,256,173 (Schmitt/Wittner) describes a sidewall lining made of a honeycomb matrix of coke and pitch in which particulate silicon carbide is embedded. U.S. Pat. No. 5,876,584 (Cortellini) discloses sidewall lining material of silicon carbide, silicon nitride or boron carbide having a density of at least 95% and no apparent porosity.
Sidewalls of known ledgeless cells are most exposed to erosion at the interface between the molten electrolyte and the molten aluminium which accumulates on the bottom of the cell. Despite formation of an inert film of aluminium oxide around the molten aluminium metal, cryolite operates as a catalyst which dissolves the protective aluminium oxide film at the aluminium/cryolite interface, allowing the molten aluminium metal to wet the sidewalls along the molten aluminium level. As opposed to aluminium oxide, the oxide-free aluminium metal is reactive at the cell operating temperature and combines with constituents of the sidewalls, which leads to rapid erosion of the sidewalls about the molten aluminium level.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for avoiding cell sidewall erosion caused by reaction with molten aluminium metal.
An object of the invention is to provide a design for an aluminium electrowinning cell in which electrolyte is inhibited from freezing on the sidewalls.
Another object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to an increased sidewall lifetime.
A further object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to a reduced erosion, oxidation or corrosion of the sidewalls.
A major object of the invention is to provide a drained cathode cell configuration with sidewalls resistant to molten electrolyte, in particular carbide and/or nitride-containing sidewalls, for crustless or substantially crustless operation.
One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The drained-cathode cell has a cell bottom which comprises an arrangement for collecting product aluminium and a peripheral upper surface that surrounds the arrangement for collecting product aluminium. At least the part of the cell bottom which is in contact with molten aluminium during operation is made of material resistant to molten aluminium.
Aluminium is produced on at least one drained cathode surface from which the produced aluminium drains into said arrangement for collecting the product aluminium during operation.
The drained-cathode cell further comprises one or more thermic insulating sidewalls extending generally vertically upwards from the peripheral surface of the cell bottom to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium. Above the peripheral surface, the or each thermic insulating sidewall is lined with a sidewall lining made of material resistant to molten electrolyte but liable to react with molten aluminium. the or each thermic insulating sidewall inhibits formation of an electrolyte crust or ledge on the sidewall lining that during operation remains permanently exposed to molten electrolyte above and around said peripheral surface.
The peripheral surface of the cell bottom is arranged to keep molten aluminium away from the sidewall lining above and around the entire peripheral surface, whereby the molten aluminium is prevented from reacting with the sidewall lining above and around the entire peripheral surface.
The drained-cathode cell design according to the invention thus keeps the molten aluminium away from all cell sidewalls preventing it from contacting and reacting with the sidewall lining resistant to molten electrolyte, enabling use of a sidewall lining made of a carbide and/or a nitride, such as silicon carbide, silicon nitride or boron nitride, without risk of damage to the sidewall lining by reaction with molten aluminium as can occur with known designs.
Usually the cell comprises four of the above mentioned insulated sidewalls in a generally rectangular arrangement. However, the invention can also be implemented with other sidewall configurations.
The upper surface of the cell bottom for example comprises opposed sloping surfaces leading from opposed sidewalls down into a central channel for the continuous removal of product aluminium, the central channel extending parallel to said opposed sidewalls. This central draining channel (or a side channel or several channels in other embodiments) preferably leads into an aluminium storage sump or space which is internal or external to the cell and from which the aluminium can be tapped from time to time.
Alternatively, the upper surface of the cell bottom comprises a series of oppositely sloping surfaces forming therebetween recesses or channels that extend parallel to opposed sidewalls. The recesses or channels can be of various shapes, for example generally V-shaped.
Usually, the peripheral surface slopes down to a flat or sloping main surface of the cell bottom which forms the drained cathode surface or which receives produced aluminium from a drained cathode surface located thereabove. This main surface leads into the arrangement for collecting product aluminium.
When the main surface is at a slope, the peripheral surface is usually inclined at a steeper slope than the main surface.
In one embodiment, the main surface comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls. The converging surfaces are inclined along second opposed sidewalls. The peripheral surface extends horizontally along the first opposed sidewalls and follows the inclination of the converging surfaces along the second opposed sidewalls. In this embodiment, the sloping peripheral surface can be of substantially uniform width around the entire cell bottom.
In another embodiment, where the main surface also comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, the converging surfaces are inclined along second opposed sidewalls, and the peripheral surface extends horizontally along the first and second opposed sidewalls, the sloping peripheral surface extends down to the converging inclined surfaces around the entire cell bottom. Usually, the sloping peripheral surface is of uniform width along the first opposed sidewalls and of non-uniform width along the second opposed sidewalls where it forms generally triangular surfaces whose sides follow the second opposed sidewalls and the converging inclined surfaces.
In a further embodiment, where the main surface also comprises downwardly converging inclined, surfaces sloping down from first opposed sidewalls, the converging surfaces are inclined along second opposed sidewalls, and the sloping peripheral surface extends horizontally along the first and second opposed sidewalls, the sloping peripheral surface is connected by at least one substantially vertical connecting wall to the main surface, i.e. at least to the converging inclined surfaces. Such connecting wall(s) is/are resistant to molten aluminium.
Usually, the drained surface(s) is/are on one or more cathodes which are part of the cell bottom and so arranged that molten aluminium produced thereon drains away from the sidewall lining into the arrangement for collecting molten aluminium. Alternatively, the drained cathode surface(s) can be on one or more cathodes located above the cell bottom, the molten aluminium draining from the cathodes onto the cell bottom and then into the arrangement for collecting molten aluminium.
The cathode and/or the cell bottom can be made of carbonaceous material, such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Pat. No. 5,362,366 (de Nora/Sekhar), prebaked carbon blocks, or graphite blocks, plates or tiles. Other suitable cathode materials which can also be used for the cell bottom are described in WO98/53120 (Berclaz/de Nora) and WO99/02764 (de Nora/Duruz).
The cathode and the cell bottom most preferably has/have an upper surface which is aluminium-wettable, for example the upper surface of the cathode or the cell bottom is coated with a coating of refractory aluminium wettable material as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar) or WO98/17842 (Sekhar/Duruz/Liu). The aluminium-wettable surface usually comprises a refractory boride, in particular TiB2, advantageously applied as a coating from a slurry of particles of the refractory boride or other aluminium-wettable material.
This aluminium-wettable surface can be obtained by applying a top layer of refractory aluminium-wettable material over the upper surface (which can already have a precoating of the refractory aluminium wettable material) and over parts of the cell surrounding the cathode.
In one embodiment in which the cathode is part of the cell bottom, the electric current to the cathode, in particular a cathode mass, may arrive through an inner cathode holder shell or plate placed between the cathode and the outer shell, usually made of steel, as disclosed in WO98/53120 (Berclaz/de Nora).
The sidewall lining can be made of tiles containing carbide and/or nitride and/or can comprise a carbide and/or nitride based coating which during cell operation is in contact with the product aluminium.
Alternatively, the sidewall lining may be coated and/or impregnated with one or more phosphates of aluminium, as disclosed in U.S. Pat. No. 5,534,130 (Sekhar) The phosphates of aluminium may be selected from: monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, and aluminium metaphosphate.
The cells according to the invention can make use of traditional consumable prebaked carbon anodes, continuously-fed Sxc3x8derberg-type anodes, as well as non-consumable or substantially non-consumable anodes.
Non-consumable anodes may comprise an electrochemically active structure made of a series of horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved. The anode members may be in a parallel arrangement connected by at least one connecting cross-member or in a concentric arrangement connected by at least one generally radial connecting member as described in WO00/40781 and WO00/40782 (both in the name of de Nora).
Suitable materials for oxygen-evolving anodes include iron and nickel based alloys which may be heat-treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), PCT/IB99/01976 (Duruz/de Nora) and PCT/IB99/01977 (de Nora/Duruz). Further oxygen-evolving anode materials are disclosed in WO99/36593, WO99/36594, WO00/06801, WO00/06805, PCT/IB00/00028 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591 and WO99/36592 (both in the name of de Nora).
Whether consumable prebaked anodes or non-consumable anodes are used, it is advantageous to preheat each anode before it is installed in the cell during operation, in replacement of a carbon anode which has been substantially consumed, or a non-consumable anode that has become disactivated or requires servicing. By preheating the anodes, disturbances in cell operation due to local cooling are avoided as when an electrolyte crust is formed whereby part of the anode is not active until the electrolyte crust has melted.
The invention also relates to a cell trough for containing molten electrolyte and product aluminium, having a cell bottom fitted with insulating cell sidewalls which are protected with a molten electrolyte resistant lining as described above.
A further aspect of the invention relates to a method of producing aluminium using the cell as outlined above which contains alumina dissolved in a fluoride-containing molten electrolyte. The method involves electrolysing the dissolved alumina to produce aluminium on the or each drained cathode surface and draining the produced aluminium from the or each drained cathode surface into the arrangement for collecting the product aluminium, the produced aluminium being kept from contacting and reacting with the sidewall lining above and around the entire peripheral surface.
Advantageously, the surface of the cell bottom is maintained at a temperature corresponding to a paste state of the electrolyte whereby the cell bottom is protected from chemical attack. For example, when the cryolite-based electrolyte is at about 950xc2x0 C., the surface of the cell bottom can be cooled by about 30xc2x0 C., whereby the electrolyte contacting the cathode surface forms a viscous paste which protects the cell bottom.